1. Field of the Invention
The present invention relates to a method for making a color filter substrate used in color televisions and personal computers, to a color filter substrate produced by the method, and to a liquid crystal device using the color filter substrate.
2. Description of the Related Art
Demand for liquid crystal displays and in particular color liquid crystal displays is increasing with the spread of personal computers, particularly portable personal computers. Cost reduction of liquid crystal displays is essential for further spread. In particular, the cost of color filters is a considerable percentage of the cost of liquid crystals and reduction of the cost would be desirable.
Various methods have been proposed for achieving this requirement while providing satisfactory color filter characteristics, as follows.
A first method is a pigment dispersion method. A photosensitive layer of a dispersed pigment is formed on a substrate and patterned to form a color pattern. These steps are repeated three times to form R, G, and B color filters.
A second method is a dying method. A water-soluble polymer layer is formed on a substrate and is patterned into a desired shape by a photolithographic process. The resulting pattern is dipped into a dye bath to color the pattern. These steps are repeated three times to form R, G, and B color filters.
A third method is an electrodeposition method. A transparent electrode is patterned into a substrate and is dipped into an electrodeposition solution containing a pigment, a resin, and an electrolytic solvent. These steps are repeated three times to form R, G, and B color filters. The color filters are finally baked.
A fourth method is a printing method. A pigment is dispersed into a thermosetting resin and the dispersion is printed. These steps are repeated three times to form R, G, and B layers. The thermosetting resin is cured by heat to form color filters.
In the first to fourth methods, a protective film is generally formed on the color filter.
In these methods, the same steps must be repeated three times to form R, G, and B layers, resulting in an increase in process costs. Such an increase in steps results in a decrease in production yield. In the electrodeposition method, the shape of the formable pattern is limited and thus is not applicable to TFT liquid crystal devices. Since the printing method has low image resolution, it is not suitable for fine-pitch patterns.
Methods for making color filters which solve these problems are ink-jet methods as disclosed in Japanese Patent Laid-Open Nos. 59-75205, 63-235901, and 1-217302. These methods, however, are still unsatisfactory in view of practical use.
Particularly, in a conventional method for imparting an ink to an ink receptive layer having low heat and solvent resistance, the ITO forming step and the alignment film forming step have process restriction. Thus, highly reliable liquid crystal devices are formed with difficulty by this method. A method for solving such a problem is the formation of a color filter by placing a curable ink directly on a substrate. FIGS. 3A to 3D are cross-sectional views of steps in this method. With reference to FIG. 3A, barriers 2 are formed on a transparent substrate 1. A curable ink 4 is charged in regions surrounded with the barriers 2. With reference to FIG. 3B, the curable ink 4 is cured by light irradiation or heat treatment to form color filters 5. With reference to FIG. 3C, a protective layer 6 may be formed, if necessary.
All of the regions surrounded with the barriers 2 must be filled with the curable ink 4. The curable ink 4 generally has a curved surface in each region because of surface tension, as shown in FIG. 3b. The solvent in the ink is evaporated in the subsequent ink-drying step. The residual solid components including a coloring agent, a resin, and additives form a color filter composed of a cured colored composition by curing. The curvature of the ink surface changes with the evaporation of the solvent, and the final curvature determines the shape of the color filter surface. A large final curvature causes a large difference in thickness in the color filter in some cases. The uneven thickness in the color filter directly reflects an uneven color density of the color filter. Thus, the uneven thickness in the color filter must be suppressed as much as possible in order to maintain superior display characteristics.